Optical lenses, such as ophthalmic lenses, are often fabricated by molding organic materials, such as plastics, for reasons including, for example, weight reduction relative to lenses fabricated from inorganic materials, such as silica glass. For individuals with presbyopia, ophthalmic lenses can be designed and fabricated to include more than one lens power within each lens, and are typically referred to as multifocal lenses. In some instances, the multifocal lens is a segmented multifocal lens in which the front surface thereof includes at least one segmented optical power addition portion that extends outward relative to the front surface of the lens.
Since optical lenses, such as ophthalmic lenses, are sometimes worn under conditions that involve exposure to actinic radiation, such as sunlight, it is desirable to provide the lenses with shading. Such shading can be provided by static dyes that do not change the level of shading and/or color with increased or decreased exposure to actinic radiation. The static dyes can be provided in the form of a coating over one or more surfaces of the optical lens. Optical lenses that are provided with shading by static dyes are generally not well suited for use under low light conditions, such as indoors, because they provide too much shading under such conditions. Photochromic compounds, in contrast to static dyes, are capable of reversibly changing the level of shading and/or color with increased or decreased exposure to actinic radiation.
In response to certain wavelengths of electromagnetic radiation (or “actinic radiation”), photochromic compounds, such as indeno-fused naphthopyrans, typically undergo a transformation from one form or state to another form, with each form having a characteristic or distinguishable absorption spectrum associated therewith. Typically, upon exposure to actinic radiation, many photochromic compounds are transformed from a closed-form, which corresponds to an unactivated (or bleached, e.g., substantially colorless) state of the photochromic compound, to an open-form, which corresponds to an activated (or colored) state of the photochromic compound. In the absence of exposure to actinic radiation, such photochromic compounds are reversibly transformed from the activated (or colored) state, back to the unactivated (or bleached) state. Compositions and articles, such as optical lenses, that contain photochromic compounds or have photochromic compounds applied thereto (e.g., in form of a photochromic coating composition) typically display colorless (e.g., clear) and colored states that correspond to the colorless and colored states of the photochromic compounds contained therein or applied thereto.
With some applications, photochromic compounds are provided in the form of a photochromic coating on the front surface of an optical lens, since the front surface typically faces and is directly exposed to the source of actinic radiation, such as sunlight. Typically, the rear surface of an optical lens blank is ground to introduce a desired optical correction or ophthalmic prescription into the lens. Providing a photochromic coating on the front surface of the lens blank allows for grinding of the rear surface without damaging the photochromic coating.
With segmented multifocal lens in which the front surface thereof includes at least one segmented optical power addition portion that extends outward relative to the front surface of the lens, applying a photochromic coating to the front surface results, in some instances, in the formation of defects due to a lack of uniform coating thickness associated with the optical power addition portion. Such a lack of uniform coating thickness can result in the undesirable formation of line defects including, for example, darker lines and/or lighter lines, when the coated optical lens is exposed to actinic radiation. Ophthalmic articles with such coating defects typically do not meet minimal required cosmetic standards.
It would be desirable to develop new methods of forming photochromic segmented multifocal lenses. It would also be desirable that such newly developed methods minimize or eliminate defects associated with non-uniform coating thicknesses on and around the multifocal segment where such defects can lead to unacceptable optical distortions.